Check for l10n cache before sync-wikiversions
Abort sync-wikiversions if critical files are missing.
This fixes T100573 by testing for the existence of:
- STAGE/php-$VERSION/cache/l10n/l10n_cache-en.cdb
- STAGE/wmf-config/ExtensionMessages-$VERSION.php
Bug: T100573
Change-Id: I50adefa19c0e3916d25703d78934b743a5f64da7